Thermal and photo-initiation curing system of photopolymer resin for 3D printing

ABSTRACT

Photopolymer resin formulation for 3D printing enhancing the performance of 3D printed objects. In one example, additives comprising two photoinitiators with different absorption spectrums are used to achieve printing and post-curing processes. A thermal initiator can also be used to complete the post-curing process by baking at appropriate temperature. Additive combinations balanced to absorb some light in the near UV range of about 400 nm to about 420 nm absorption wavelength band or spectrum while fluorescing light at frequencies higher than their absorption wavelength allowing the creation of clear transparent materials using only near UV light sources (i.e., about 400 nm to about 420 nm) instead of deep UV (&lt;400 nm) light sources.

CROSS-REFERENCE

This application claims priority to and is a Divisional application ofU.S. patent application Ser. No. 14/806,508 filed Jul. 22, 2015 whichclaims priority to U.S. Patent Application No. 62/027,493 filed Jul. 22,2014 both of which is incorporated herein for any and all purposes.

FIELD OF THE INVENTION

The embodiments of the present invention relate to additive use inphotopolymer resin formulations for 3D printing systems, including, butnot limited to, stereolithography.

BACKGROUND OF THE INVENTION

Three dimensional (3D) printing or additive manufacturing is a processin which a 3D digital model is manufactured by the accretion ofconstruction material. The 3D printed object is created by utilizing thecomputer-aided design (CAD) data of an object through sequentialconstruction of two dimensional (2D) layers or slices that correspond tocross-sections of 3D objects. Stereolithography (SL) is one type ofadditive manufacturing where a liquid resin is hardened by selectiveexposure to a radiation to form each 2D layer. The radiation can be inthe form of electromagnetic waves or an electron beam. The most commonlyapplied energy source is ultraviolet, visible or infrared radiation. Theliquid photopolymer resin can contain monomers, oligomers, fillers andadditives such as photoinitiators, blockers, colorants and other typesdepending on the targeted properties of the resin.

In the past, the focus of the SL resins has been in the deep range atabout 355 nm. These sources work well and there are many formulationsfor these sources. However, lasers at 355 nm are extremely expensiverelying on frequency tripled YVO4 laser crystal technology. Furthermore,DLP projectors are typically unreliable due to UV breakdown the fartheraway from visible light and are not generally compatible withfrequencies below 400 nm. Due to the recent commercialization of Blu-raylaser diodes capable of directly emitting at 405 nm and production of400 nm to 420 nm direct violet LEDs used in the production of whitelight bulbs leading to low cost light sources, there has been increasedinterest in creating SL resins that can function with near UV sources inthe about 400 nm to about 420 nm range.

One challenge encountered with SL is the incomplete curing of theprinted 3D object including the surface and interior of the printed 3Dobjects. If the 3D object is cured completely during the 3D printingprocess, the interlayer adhesion is too weak and the print may fail. Inaddition, the material may stick to parts of the printing apparatus andnot release properly. Hence, it is desirable to cure only in the rangeof (5% to 99%) and not 100% during the printing process. Afterwards, theuncured resin needs to be removed from the surface and the remainingresin cured to a higher rate. The uncured liquid resin on the surface ofthe printed 3D objects can be mostly removed by washing with solvents.However, the uncured resin inside the printed 3D object is difficult toremove. Uncured resin inside is undesirable for a few reasons. First,uncured liquid resin leaking from the printed 3D objects may causehealth problems to end users because the liquid resin may containreactive chemicals. Second, the printed 3D objects do not reach optimalmechanical performance because the uncured liquid resin may soften theobject. Third, the uncured resin may cause problems in some industrialapplications of the objects where high chemical inertia is required.

In order to fully cure the printed objects, the current art is blendingexcess photoinitiator and post-curing the parts at similar wavelengthlight used in the printing process. This solution can be problematicbecause the excess photoinitiator can cause yellowing in transparentprinted parts.

Another challenge encountered with SL is creating clear transparentresin using light in the near UV/violet range. Since the near UV/violetlight needs to be absorbed to be reacted with the liquid resin, thisviolet light is normally removed from the spectrum that can pass throughresulting in a yellow appearance.

It would be advantageous to use certain types of additives inphotopolymer resin formulations to overcome challenges noted above aswell as enhancing the performance of the printed objects without causingan increase in the costs.

SUMMARY

Accordingly, the embodiments of present invention relate to additivemanufacturing. More particularly, the embodiments of the presentinvention relate to SL. Even more particularly, the embodiments of thepresent invention relate to photopolymer resins used with SL. Theembodiments of the present invention demonstrate enhanced performance ofprinted objects by the proper use of additives in liquid resinformulations for stereolithography 3D printing systems.

Two of the embodiments of the present invention overcome the above-notedfirst challenge by using two photoinitiators or one photoinitiator andone thermal initiator in the photopolymer resin formulation. Such dualinitiation system photopolymer resin formulations allow printed objectsto be fully cured in post-curing without losing high performance of theobjects.

It is well recognized in the field of liquid photopolymer resins thatadditives like light blockers are used to finely control the curingthickness of printed layers to achieve the required detail printing. Theused blockers can be colorants such as pigments carbon black, blue, red,UV blockers or fluorescent agents/optical brightener. Colorants and UVblockers typically absorb the light and do not retransmit light.Fluorescent agents/optical brighteners absorb light and emit light at adifferent frequency. When the light absorbed is invisible UV light andthe transmitted is in the visible range, the object will appearoptically brighter since more visible light is present.

A third embodiment of the present invention uses certain additives tofinely control the curing thickness while overcoming the above-notedsecond challenge by using certain types of additives which work as bothlight blocker and brightener. In this embodiment, high performance clearprinted objects are achieved by using additives which acts as both lightblocker and brightener.

Other variations, embodiments, and features of the present inventionwill become evident from the following detailed description, drawingsand claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an absorption spectrum of Irgacure TPO (CAS:75980-60-8), Irgacure 1173 (CAS: 7473-98-5) and emission of light sourceof the 3D printer;

FIG. 2 illustrates an absorption and fluorescence emission spectra ofBenetex OB Plus (CAS: 7128-64-5) and emission of light source of the 3Dprinter;

FIG. 3 illustrates a flowchart detailing a dual photoinitiator processaccording to the embodiments of the present invention; and

FIG. 4 illustrates a flowchart detailing a photoinitiator and thermalinitiator process according to the embodiments of the present invention.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles inaccordance with the embodiments of the present invention, reference willnow be made to the embodiments illustrated in the drawings and specificlanguage will be used to describe the same. It will nevertheless beunderstood that no limitation of the scope of the invention is therebyintended. Any alterations and further modifications of the inventivefeature illustrated herein, and any additional applications of theprinciples of the invention as illustrated herein, which would normallyoccur to one skilled in the relevant art and having possession of thisdisclosure, are to be considered within the scope of the inventionclaimed.

A few of the advantages of the embodiments of the present inventioninclude: (i) minimizing the hazardous chemicals in printed 3D objects ata low cost; (ii) easy operation for manufacturing large scalephotopolymer resin products and (iii) significantly enhancing theperformance of printed objects.

Referring now to the first embodiment of the present invention, Table 1details an exemplary dual photoinitiator resin formulation having about68.50% by weight of EBECRYL 4858 (CAS: 120146-73-8) (a low viscosityaliphatic urethane diacrylate) (an oligomer), about 24.49% by weight ofDPGDA (CAS: 57472-68-1) (dipropylene glycol diacrylate) (a monomer),about 4.95% by weight of EBECRYL 113 (CAS: 1204322-63-3) (a low odormonofunctional acrylated aliphatic epoxy) (a second monomer), about0.98% by weight of Irgacure TPO (CAS: 75980-60-8) (acyl phosphine oxidephotoinitiator), about 0.98% by weight of Irgacure 1173 (CAS: 7473-98-5)(2-Hydroxy-2-methyl-1-phenyl-propan-1-one) (a second photoinitiator) andabout 0.10% weight of Benetex OB Plus (CAS: 7128-64-5) (a blocker)(2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole).

TABLE 1 Concentration Materials (% by weight) Function EBECRYL 4858(CAS: 120146-73-8) 68.50 Oligomer DPGDA (CAS: 57472-68-1) 24.49 MonomerEBECRYL 113 (CAS: 1204322-63-3) 4.95 Monomer Irgacure TPO (CAS:75980-60-8) 0.98 Photoinitiator Irgacure 1173 (CAS: 7473-98-5) 0.98Photoinitiator Benetex OB Plus (CAS: 7128-64-5) 0.10 Blocker

In further detail, still referring to the embodiments of the presentinvention as set forth in Table 1, two photoinitiators (Irgacure TPO andIrgacure 1173) with distinct absorption spectra shown in chart 100 ofFIG. 1 are used. During the 3D printing process, only the firstphotoinitiator Irgacure TPO (CAS: 75980-60-8) with significantabsorption wavelength starting from about 420 nm to lower wavelengths isinitiated to start the polymerization process because its absorptionband overlaps the wavelength range of the light source with a peak atabout 405 nm. During a post-curing process, a light source with a muchlower wavelength initiates the second photoinitiator Irgacure 1173 (CAS:7473-98-5) with an adsorption wavelength lower than 380 nm which isoutside the available wavelengths during the printing process so do notaffect the printing quality. It is obvious that variations can be madeby using different first photoinitiators and/or second photoinitiatorswherein the second photoinitiator has no absorption of the light sourceused in the 3D printing process. The dual photoinitiator system allowsthe majority of the resin to be hardened in printing by using onephotoinitiator and then fully hardened in post-curing by using anotherphotoinitiator. In addition, a thermal initiator may be added as well(see Table 2). Thermal initiators allow deep post curing withoutaffecting the resin during regular printing.

Referring now to the second embodiment of the present invention, Table 2details an exemplary resin formulation having one photoinitiator and onethermal initiator. More particularly, a resin formulation having about68.50% by weight of EBECRYL 4858 (CAS: 120146-73-8) (an oligomer), about24.49% by weight of DPGDA (CAS: 57472-68-1) (a monomer), about 4.95% byweight of EBECRYL 113 (CAS: 1204322-63-3) (a second monomer), about0.98% by weight of Irgacure TPO (CAS: 75980-60-8) (a photoinitiator),about 0.98% by weight of benzoyl peroxide (a thermal initiator) andabout 0.10% weight of Benetex OB Plus (CAS: 7128-64-5) (a blocker).

TABLE 2 Concentration Materials (% by weight) Function EBECRYL 4858(CAS: 120146-73-8) 68.50 Oligomer DPGDA (CAS: 57472-68-1) 24.49 MonomerEBECRYL 113 (CAS: 1204322-63-3) 4.95 Monomer Irgacure TPO (CAS:75980-60-8) 0.98 Photoinitiator Benzoyl peroxide 0.98 Thermal InitiatorBenetex OB Plus (CAS: 7128-64-5) 0.10 Colorant

In further detail, still referring to the embodiments of the presentinvention as set forth in Table 2, the dual initiation system uses twodifferent initiation mechanisms in printing and post-curing. During the3D printing process, the photoinitiator Irgacure TPO (CAS: 75980-60-8)is initiated to start the polymerization process where thephotoinitiator is broken down to generate free radicals under exposureto the light source of the printer while post-baking at about 80 degreesCelsius to activate the thermal initiator benzoyl peroxide to fully curethe printed objects. The initiation temperature of the thermal initiatorneeds to be carefully monitored and should be higher than the peak localtemperature of resin in the printing and lower than the maximumtemperature that the printed object can maintain its performance afterbaking. It is obvious that variations can be made by using the thermalinitiator with initiation temperature in the range described above. Thisdual initiation system allows the majority of the resin to be hardenedin printing by using the photoinitiator during printing and fullyhardened in post-curing by post-baking.

Referring now to third embodiment of the present invention, Table 3details an exemplary resin formulation for achieving clear printedobjects containing about 69.28% by weight of EBECRYL 4858 (CAS:120146-73-8) (an oligomer), about 24.75% by weight of DPGDA (CAS:57472-68-1) (a monomer), about 4.95% by weight of EBECRYL 113 (CAS:1204322-63-3) (a second monomer), about 0.99% by weight of Irgacure TPO(CAS: 75980-60-8) (a photoinitiator) and about 0.03% by weight ofBenetex OB Plus (CAS: 7128-64-5) a light blocker and optical brightener.

TABLE 3 Concentration Materials (% by weight) Function EBECRYL 4858(CAS: 120146-73-8) 69.28 Oligomer DPGDA (CAS: 57472-68-1) 24.75 MonomerEBECRYL 113 (CAS: 1204322-63-3) 4.95 Monomer Irgacure TPO (CAS:75980-60-8) 0.99 Photoinitiator Benetex OB Plus (CAS: 7128-64-5) 0.03Blocker/ Brightener

In further detail, still referring to the embodiments of the presentinvention as set forth in Table 3, Irgacure TPO (CAS: 75980-60-8) isused as photoinitiator while Benetex OB Plus (CAS: 7128-64-5) is used asblocker and also brightener. As shown in FIG. 2, the absorption band ofBenetex OB Plus (CAS: 7128-64-5) covers mostly UV and violet light whilethe wavelength of the light source used in this embodiment is about 405nm, and the absorption spectrum of Irgacure TPO (CAS: 75980-60-8) tapersoff quickly after about 405 nm. During the printing process, light fromthe light source is partially absorbed by Benetex OB Plus (CAS:7128-64-5) and re-emitted outside the absorption of the Irgacure TPO(CAS: 75980-60-8), thus acting as a blocker. As a result, the curingthickness of one layer can be finely tuned by adjusting theconcentration of Benetex OB Plus (CAS: 7128-64-5) in the photopolymerresin formulation. Unlike traditional blockers, with Benetex OB Plus(CAS: 7128-64-5) the light that was blocked during curing, is recoveredby the fluorescence of OB Plus in the printed part (i.e., for theprinted part, UV and violet ambient light is absorbed by Benetex OB Plus(CAS: 7128-64-5) and re-emitted as higher wavelength visible light,overcoming any blocking that was necessary in the curing process. TheBenetex OB Plus (CAS: 7128-64-5) here essentially shifts the absorbed UVand violet lights to visible lights and enhances the clearness of theprinted parts, while also acting as a blocker during the printingprocess. It is obvious that variations can be made by using thedifferent additives with similar fluorescence properties.

FIG. 3 shows a flowchart 200 detailing a dual photoinitiator processaccording to the embodiments of the present invention. The liquidphotopolymer resin or dual photoinitiator resin contains two differentphoto initiators with different absorption spectra. The firstphotoinitiator has an absorption range within a range of a light sourceused in an additive manufacturing process. The first light source is thelight source of the additive manufacturing device. The firstphotoinitiator is activated by the first light source to harden or curethe printed 3D object. Depending whether or not the printed objectsreach a targeted performance, post-curing may be necessary. Ifpost-curing is required for the dual photoinitiator resin system, thesecond photoinitiator, as described above, has an absorption rangeoutside the range of the light source used by the additive manufacturingdevice. The second photoinitiator is therefore passive and was notactivated by the first light source during the additive manufacturingprocess. The second photoinitiator instead has an absorption range thatis within a wavelength of a second light source and is exposed to thesecond light source after the completion of the additive manufacturingprocess. During post-curing, the 3D printed part is exposed to thesecond light source after the additive manufacturing process. Exposingthe uncured photopolymer resin to the second light source initiates apolymerization process with the second photoinitiator. The absorptionrange of the second photoinitiator may cover the wavelength of thesecond light source but not the first light source.

Accordingly, at 205, the additive manufacturing device contains theresin having dual photoinitiators. At 210, it is determined is theadditive manufacturing device is setup. If so, at 215, the firstphotoinitiator is activated. At 220, it is determined if the printed 3Dobject meets the established high performance standard. If so, at 225,the printed 3D object is cleaned and finished. If the printed 3D objectdoes not meet the high performance standard, at 230, the printed 3Dobject is subject to the post-cure activation of the secondphotoinitiator (i.e., application of second light source) prior to beingcleaned and finished at 225.

FIG. 4 shows a flowchart 300 detailing a photoinitiator and thermalinitiator process according to the embodiments of the present invention.During the printing process, the photoinitiator has an absorption rangewithin a range of a light source used in an additive manufacturingprocess. The first light source is the light source of the additivemanufacturing device. Only the photoinitiator is activated by the firstlight source to harden or cure the printed 3D object. Depending onwhether or not the printed objects reach a pre-established targetedperformance, post-curing may be necessary. If post-curing is requiredfor the dual initiation resin system, the thermal initiator, asdescribed above, is activated within a specific temperature rangeoutside of the temperature range of encountered during the additivemanufacturing process. Accordingly, the photopolymer resin is exposed tothe first light source during the additive manufacturing process toinitiate a reaction with the photoinitiator. During post-curing, the 3Dprinted part is exposed to a baking process whereby the 3D printedobject is subjected to a temperature or temperatures which cause thethermal initiator to be activated to finalize the curing process.

Accordingly, at 305, the additive manufacturing device contains theresin having dual photoinitiators. At 310, it is determined is theadditive manufacturing device is setup. If so, at 315, thephotoinitiator is activated. At 320, it is determined if the printed 3Dobject meets the established high performance standard. If so, at 325,the printed 3D object is cleaned and finished. If the printed 3D objectdoes not meet the high performance standard, at 330, the printed 3Dobject is subject to the post-cure activation of the thermal initiator(i.e., baking to a temperature range) prior to being cleaned andfinished at 325.

While the foregoing written description of the embodiments of thepresent invention enable one of ordinary skill to make and use what isconsidered presently to be the best mode thereof, those of ordinaryskill will understand and appreciate the existence of variations,combinations, and equivalents of the specific embodiment, method, andexamples herein. The invention herein should therefore not be limited bythe above-described embodiments, methods, and examples, but by allembodiments and methods within the scope and spirit of the invention asclaimed.

We claim:
 1. A 3D printing resin formulation comprising: about 0.98% byweight of a photoinitiator in the form of acyl phosphine oxide; about0.98% by weight of a thermal initiator in the form of benzoyl peroxide;about 68.50% by weight of an oligomer in the form of a low viscosityaliphatic urethane diacrylate; about 24.49% by weight of a monomer inthe form of dipropylene glycol diacrylate; and about 0.10% weight of alight blocker and optical brightener in the form of2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole).
 2. The 3D printingresin formulation of claim 1 further comprising about 4.95% by weight ofa second monomer in the form of a low odor monofunctional acrylatedaliphatic epoxy.
 3. A 3D printing resin formulation comprising about0.98% by weight of a photoinitiator in the form of acyl phosphine oxide;about 68.50% by weight of an oligomer in the form of a low viscosityaliphatic urethane diacrylate; about 24.49% by weight of a monomer inthe form of dipropylene glycol diacrylate; about 0.10% by weight of alight blocker and optical brightener in the form of2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole); and a thermalinitiator having an initiation temperature range higher than atemperature reached during a 3D printing process and lower than amaximum temperature that a printed 3D object can maintain its structureafter a post 3D printing baking process.